1. Field of the Invention
The present invention relates to image forming apparatuses equipped with a fixing device for fixing a toner image transferred to a recording medium, more specifically, relates to an image forming apparatus equipped with a non-contact type temperature detection sensor for detecting the temperature of a heating member and an unusual temperature rise preventing member (thermostat) for preventing unusual overheating of the heating member.
2. Description of the Related Art
In image forming apparatuses such as a copying machine, a printer and the like, an image carrying body such as a photosensitive body is charged, an electrostatic latent image is formed on the surface of the image carrying body by irradiating laser light, and the electrostatic latent image is visualized by a developing device whereby a toner image made up of fine toner particles is formed. After transferring the toner image onto a recording medium such as paper, the toner image on the recording medium is fixed by the fixing device and an image is formed.
The fixing device is provided with a heating roller including a heater inside a rotating body, and a pressure roller for pressurizingly contacting the recording medium to the heating roller. The recording medium formed with the toner image is passed between the heating roller and the pressure roller, so that the toner image is melted from heating and pressurization and is fixed on the recording medium. This fixing device is removable with respect to a main body of the image forming apparatus, and is removed from the main body in replacement due to its life period or in dealing with jam of the recording medium being conveyed.
In such fixing device, a non-contact type temperature sensor for detecting the temperature of the heating roller by means of infrared ray is provided to prevent unusual overheating (unusual temperature rise) of the heating roller. The heater of the heating roller is controlled based on the temperature detected by the non-contact type temperature sensor. Further, a thermostat is arranged at a position facing the heating roller, and the wiring to conduct from a power source to the heater in the heating roller is connected by way of the thermostat. The conduction from the power source is stopped by the thermostat when the heating roller is in an overheated state.
As described in Japanese Patent Application Laid-open No. 7-260579, the non-contact type temperature sensor has a window for the infrared ray to enter, and a resin film having a thermal sensitive element for detecting the infrared ray fixed thereon is arranged at the lower part of the window. A thermal sensitive element for temperature compensation is arranged adjacent to the thermal sensitive element for infrared ray detection by way of a frame body having satisfactory thermal conductivity, and a shielding part for shielding the incidence of the infrared ray is arranged on the upper part of the thermal sensitive element for temperature compensation. In such non-contact type temperature sensor, a potential difference is generated between the thermal sensitive element for infrared ray detection and the thermal sensitive element for temperature compensation according to the infrared ray that has entered from the window. The surrounding temperature is acquired by detecting the amount of infrared ray according to the potential difference.
However, when the recording medium of a small size is conveyed between the heating roller and the pressure roller, the temperature at a region on the heating roller where the recording medium has passed lowers, but the temperature at a region where the recording medium has not passed rises. The non-contact type temperature sensor detects the amount of infrared ray that has entered from the window to acquire the temperature however, it also detects the infrared ray at the periphery of the window. Thus, the temperature of the heating roller may be inaccurately detected due to a temperature difference on the heating roller, which may cause malfunction of the thermostat.